1. Field of the Invention
The invention is based on and relates to an arrangement for generating a digitally modulated test signal that is generated in the form of a large number of transmission channels from a digital modulation data flow according to a predetermined digital modulation standard and is fed as I and Q values to an I/Q modulator.
2. Description of the Related Art
Digitally modulated high-frequency or baseband test signals, which are generated in signal generators, are often required for measurement purposes. An enormously wide variety of digital modulation processes have now come into use and depending on these and the measurement tasks, signal generators of this kind operate by one of the following methods of signal processing.
In the first method, a modulation data stream generated internally or externally in a data source is converted into I and Q values by coding and mapping (set of rules which assigns I and Q values to each modulation symbol as a function of the particular complex type of modulation) in a modulation coder and is then fed to the downstream IQ modulator, whose output signal is then converted to the desired high frequency (the Rohde & Schwarz company's SMIQ vector signal generator, data sheet PD757.4582 and extract from associated manual 1084.80004.03, pages 2.78 to 2.112). The internal data source used may for example be a memory from which the data is read out. It is also known for any desired complex data sequences to be assembled from stored data by means of a signal processor, an example of such data sequences being so-called TDMA signals such as are used for global mobile telephone systems (see the various digital modulation standards which are possible, as listed on p. 8 of the data sheet for the SMIQ signal generator). The data sequences that are assembled internally or externally in this way can be processed directly in real time. They may however also be buffer-stored in a memory and only then fed to the IQ modulator.
A second method of signal processing comprises calculating the I and Q values and storing the sequence of I/Q values that has been calculated in this way in w memory for I and Q and then reading the digital IQ values out from this memory, converting them into analog signals and then feeding them directly, in filtered form, to the IQ modulator (a two-channel ARB generator, e.g. the Rohde & Schwarz company's AMIQ modulation generator, data sheet PD757.3970.12 and associated unit specification 1110.3339.11, pages 4.1 to 4.14). This second method is suited above all to modulation standards where a large number of individual transmission channels are generated simultaneously, as has now become standard practice on many modern-day mobile telephone networks. Under the so-called CDMA standard, 64 transmission channels are for example generated simultaneously by means of a so-called Walsh code (as described in, for example, “North American Cellular CDMA”, Hewlett-Packard Journal, December 1999, pages 90 to 97), while under the up-to-date W-CDMA method (see description in the AMIQ data sheet, page 9) there are even up to 512 individual transmission channels generated, which are modulated simultaneously onto one or more carriers. This second method does however have the disadvantage that it is not possible to operate in real time, i.e. pre-calculated signals have to be used and external data supplied by the user cannot be made use of. The pre-calculated items of data have to be stored in a memory at a limited length and can therefore only be assembled into a data sequence of any length by being repeated a multiplicity of times in succession. There are therefore limits to how far measurements where the content of the data is crucial can be made, such measurements being required for example for so-called BER (bit error rate) measurement (see the AMIQ data sheet, page 5). For synchronization with data on higher layers (under the ISO layer model) as well or for decoding tests, it is necessary for a lengthy data sequence to have the correct data content or for data made available from outside to be processed in real time.
Hence, although signal generators that operate by the second method do generate complex signals with the correct spectrum and the correct signal statistics, there is, because of the limited storage length, a limit to how far measurements where the correct content of the data is crucial can be made.
Signal generators that operate by the first method on the other hand are, it is true, suitable for making measurements where the correct content of the data is crucial, but they cannot be used for modulation standards under which a large number of transmission channels are used, because the cost and complication required for this purpose would be unacceptable and the performance of normal computers would not be good enough for it. Signal generators that operate by the first method are therefore so far being used only for measuring a few, e.g. four, channels (e.g. for BER measurements, synchronization with data on higher layers, decoding tests) and the remaining channels, such as the remaining 508 channels in the case of W-CDMA for example, are left unused. Such measurements are thus not a true reflection of reality because the neighboring channels too have an effect on the measured result. Nor does adding noise as a substitute for the neighboring channels give a true reflection of reality because noise does not give the orthogonality between the channels that is required under the standard and hence affects the reception characteristics and the measured results are thus falsified.
Hence, although measurements on receivers can be made in real time with signal generators that operate by the first method, the conditions are not a true reflection of reality because only some of the channels are busy and the remaining channels are missing.